Although a considerable amount of work has been done on control of liver glycogenolysis, few studies are available that attempt to define which of the many proposed control mechanisms are operative in the live animal during exercise or stress when liver glycogen stores can be utilized very rapidly. The purpose of these investigations is to determine which factors are most important in controlling liver glycogenolysis in the exercising animal and to determine how these factors are influenced by endurance exercise training. The first step will be to study the response of plasma insulin, glucagon, epinephrine, norepinephrine, glucose, FFA, and lactate to a wide spectrum of intensities and durations of exercise and to correlate hormone and metabolite levels with rates of liver glycogen utilization and liver cAMP in rats. Liver glycogen is utilized at a slower rate in endurance-trained animals than in non-trained animals. The hormonal adaptations to training will therefore be correlated with the slower rate of glycogen utilization during bouts of exercise which produce rapid rates of liver glycogenolysis in non-trained rats, but which have little effect on glycogenolysis in livers of trained rats. The relative roles of glucagon, caecholamines and the sympathetic innervation of the liver in exercise-induced liver glycogenolysis will be examined.